Injection molding of plastic parts is a common manufacturing practice. Various articles of commercial value such as plastic bottles, toothbrushes, and children's toys, are made using well-known injection molding techniques. Injection molding generally involves melting plastic then forcing the melt stream at high temperatures and pressures through one or more gates into a mold cavity. The melt cools in the shape of the mold cavity, which is opened to eject the finished part.
A valve gated injection molding apparatus is well known, as shown and described in U.S. Pat. No. 4,380,426 to Gellert, incorporated herein in its entirety by reference thereto. Usually a valve pin has a cylindrical or tapered front end and reciprocates between a retracted open position and a forward closed position in which the front end is seated in a gate. In some applications, the valve pin functions in the reverse direction and closes in the retracted position.
Valve-gated mechanisms are, however, typically designed to open and close the gates in a binary fashion, i.e., the gate is either open or it is closed without allowing for a partially opened scenario in which the melt flow rate or amount is controlled. In some manufacturing processes, the ability to control the melt stream during the shot is highly desirable. For example, in a multi-gated system, wherein a single mold cavity is fed melt through multiple gates, a common manifold serves all of the gates. However, a “knit line” is formed at the interface where melt flowing from one gate meets melt flowing from another gate. Even though all of the gates are commonly fed, the ability to control the flow rate through each gate individually would allow the designer to control the location of the knit line for structural or aesthetic purposes.
Another instance in which control over the melt stream flow is desirable is when a number of parts are simultaneously molded. Each mold cavity is fed melt by an individual gate. However, the mold cavities are not necessarily all the same size, such as when components of an interlocking piece are simultaneously molded, as in the sections of a cellular telephone casing or the base and cover of a packaging system. The common melt stream is important, so that the plastic characteristics are as uniform as possible between the pieces; however, as the pieces are not of a uniform size, one mold customarily takes longer to fill than the other(s). If the larger mold cavity could be filled more quickly, then both parts would be ready for ejection from the mold at the same time.
Various methods exist in the art to provide this type of control over the melt stream. The gates could be individually re-tooled for every new product, but this is expensive and time-consuming. U.S. Pat. No. 5,556,582 to Kazmer et al., incorporated herein in its entirety by reference thereto, describes a system wherein an adjustable valve pin is located in the gate, which is located in the manifold. The valve pin can be dynamically adjusted by a computer according to pressure data read at or near the injection point in the mold. The valve pin has a tapered head and the melt channel has a complementary geometry, such that the melt stream is slowed to an eventual full stop. If multiple valves are used, each is independently controlled. A hot runner nozzle is not provided. Also, as the system is used, the repetitive action of the valve pin produces significant wear on the tip of the valve pin. This wear, a result of repeated impact with the mold cavity, eventually reduces the cross-sectional diameter of the tip of the valve pin. As the tip of the valve pin is also used for flow control purposes, the ability of the system to control the flow effectively is diminished or eliminated over time.
Another system is described in U.S. Patent Application Pub. No. 2002/0121713 to Moss et al., incorporated herein in its entirety by reference thereto. In this publication, a valve pin is located in the manifold, with a tapered valve pin head disposed at the inlet point to a hot runner nozzle. The melt channel at the inlet point has a corresponding geometry to the tapered pin head, such that when the pin head is pushed into the inlet, the melt stream slows to an eventual stop.
Yet another system is described in PCT International Publication No. WO 01/21377 to Kazmer et al., incorporated herein in its entirety by reference thereto. In this publication, the manifold includes “shooting pot” technology. A portion of the melt stream is diverted from the manifold melt channel into a separate compartment or “well”. Disposed within this well is an actuated ram, which can be positioned to seal the opening of the well. A nozzle is located downstream of the well. The flow of melt through a mold gate orifice is controlled by an actuated valve pin. When the melt stream is introduced into the manifold melt channel, the valve pin is seated within the mold gate orifice to prevent flow into a mold cavity. The ram is located in a retracted position so that a volume of melt from the melt stream may be diverted into the well and contained therein. To start the shot, a gating mechanism located upstream from the well closes the manifold melt channel, thereby preventing the introduction of new melt into the well. The valve pin is unseated from the mold gate orifice, and the ram is moved forward at a first velocity to force melt into the mold cavity. A system of pressure sensors measures the pressure in the system and compares that pressure reading to a target pressure profile. If greater pressure is required, the ram velocity is increased. Alternatively, if lesser pressure is required, the ram velocity is slowed. When the ram reaches its lowermost position, the mold cavity is full, and the mold gate orifice is closed. Through this manipulation of the ram velocity, the flow rate of the melt stream can be controlled. This control over the melt stream requires completely closing off of one portion of the manifold melt channel in order to manipulate the melt stream in another portion thereof.
In many injection molding devices, the flow of the melt through the gate into the mold cavity is controlled by a valve unit. Such valve units consist in general of a linear actuator and of a valve pin, which passes through the hot runner manifold and extends up and into the sprue opening. For opening and closing the gate the valve pin is moved backward and forward by the actuator. The actuator of the valve unit is arranged at the side of the manifold opposite the mold cavity, generally above the manifold. In certain applications, the valve pin is moved by a piston arranged in the actuator, the piston being mostly driven hydraulically or pneumatically.
For controlling the movements of the actuator, the position of the valve pin can be measured, for example, by a position sensor and transmitted to a control unit. In the case where a non-contact position sensor is used, the high temperatures of the hot runner manifold makes it difficult to determine accurately the position of the valve pin with respect to the area of the gate. Further, position sensors often fail to function at higher temperatures. For example, some position sensors fail at temperatures as low as around 80° C. to around 120° C. Therefore, there is a need to protect the position sensors used in conventional valve-controlled injection molding devices from the heat generated by the hot runner manifolds